Many gardeners feel that nothing compares to homegrown tomatoes (Lycopersicon esculentum), and, unfortunately, various insects agree. Several tortoise beetle species feed on the undersides of tomato plant leaves, leaving the foliage riddled with small, round holes. Beetle feeding activity typically causes slight to moderate injuries, although large populations can cause severe damage, especially when they attack seedlings. You can get rid of tortoise beetles using mechanical, cultural and chemical control methods.
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About Tortoise Beetles
About 35 known species of tortoise beetles (Plagiometriona spp., Metriona spp.) live in North America, and most adults reach about 1/4 inch in length with round or oval bodies that look similar to ladybugs. Unlike ladybugs, however, the outer shells of tortoise beetles extend over their bodies, so they don't appear to have a head or any legs. When you take a close look at them, some species look like tiny turtles encased in clear membranes. Species vary widely in coloring, including dull red, brassy green, dark brown and bright metallic gold, and some species can even change colors when stressed or threatened.
Remove the Pests
Control small tortoise beetle populations by handpicking the slow-moving adults from plant foliage and dropping them into a pail of soapy water. If you're squeamish, wear latex gloves or use tweezers to grip the beetles. When you're finished, simply pour out the bucket's contents in an out-of-the-way section of your garden.
After the adult beetles emerge in spring to eat and mate, the females lay masses of 15 to 30 white to beige-colored eggs beneath plant leaves. Search the undersides of leaves for the masses, squishing any you find between your fingers or scraping them off the foliage with a dull knife. Search for eggs at least once a week because they only take seven to 10 days to hatch.
Spray Soap Solutions
Spraying soapy water on tomato plants kills tortoise beetle eggs and larvae while making the leaves inhospitable for adult insects. Purchase a commercial, ready-to-use insecticidal soap product or make your own solution by mixing 1 tablespoon of liquid soap for every 1 quart of water. Use pure castile soap or an environmentally friendly dish soap that doesn't contain any additives, such as perfume, whitener or moisturizer, and use a clean spray bottle. Evenly cover the tops and undersides of leaves; to work effectively, the solution must make direct contact with pests. Spray affected plant foliage every morning until beetles disappear.
Soap solutions can burn sensitive plants, so test the spray on a small section of leaves and allow it to sit for at least 24 hours before checking for damage. If no burning, browning or withering occurs, spray your entire plant.
Use Carbaryl Spray
Spraying tomato plant foliage with a carbaryl-based insecticide can help control tortoise beetle infestations. Choose a ready-to-use product or follow the mixing instructions on a bottle of concentrate. One product recommends mixing 1.5 fluid ounces of carbaryl concentrate for every 1 gallon of water. Use a small garden sprayer to completely coat the tops and undersides of foliage. Repeat applications, if necessary, waiting at least seven days between treatments. You must wait at least three days between spraying plants with carbaryl and harvesting your tomato crop.
Soap sprays aren't toxic to humans or animals, but take precautions to avoid getting the solution in your eyes. Carbaryl solutions can cause skin or eye irritation, so wear protective clothing, waterproof gloves and goggles to avoid exposure.
Insecticidal soap and carbaryl sprays can kill beneficial insects along with the pests. Spray your tomato plants in the early morning or early evening when the good bugs aren't as active. Avoid spraying blooming tomato plants to avoid harming honeybees.
Spray only on calm days when no precipitation is expected for at least 24 hours. Reapply treatments after rainfall.
Don't allow family members, including pets, around treated plants until the solution dries.
Follow Cultural Guidelines
Tortoise beetles rarely cause serious damage to healthy tomato plants, so follow the proper cultural practices to help prevent injury. Tomatoes are a warm-season crop, so cold temperatures cause stress and make plants more susceptible to feeding damage. Wait at least one week after your area's last average frost date to plant. Tomatoes need fertile, well-drained soils in locations that receive at least 8 hours of direct sunlight every day.
Before planting, work 2.5 to 3 pounds of 5-10-10 fertilizer for every 100 square feet of growing space. Work the fertilizer down into the top 12 inches of soil. When tomato fruits reach about 1 inch in diameter, feed each plant approximately 1/2 cup of 5-10-5 fertilizer and work it down into the top inch of soil; repeat application just before harvest time. Tomatoes need consistent moisture, so give plants about 1 inch of water every week if your area doesn't receive at least that amount of rainfall.
Many gardeners consider tortoise beetles to be beneficial insects because they feed on nasty, hard-to-eliminate weeds, such as horsenettle (Solanum carolinense, U.S. Department of Agriculture plant hardiness zones 4 through 11), musk thistle (Carduus nutans, USDA zones 3 through 11) and field bindweed (Convolvulus arvensis, USDA zones 3 through 9). Making your tomato plants inhospitable to the beetles, such as by spraying the plants with soap solution, can force the insects to feed on unwanted weeds instead.